The present invention relates in general to field emission devices, and in particular, to the use of a field emission cathode within a focused microwave beam traveling wave tube.
A great deal of effort was put into the design and manufacture of vacuum tube devices in the millimeter wave range: most of this work involved extension of the simple vacuum triode. This approach has inherent limitations in the push to higher frequencies. Slow wave or magnetically confined beam devices all require high magnetic fields for focusing or steering the electron beam. Most of these examples are large and heavy and are thus avoided for space-based and in-flight applications.
Listed below are various RF (radio frequency) amplifier and source technologies:
Electron Bombarded Semiconductor (EBS)
Traveling Wave Tube (TWT)
Pancake (printed circuit) TWT (PTWT), including multi-beam
Klystron and Extended Interaction Klystron (EIK)
Semiconductor (only as a baseline)
EBSxe2x80x94EBS combines vacuum and semiconductor technologies and offers high gain and bandwidth. It can be made flat in response from DC to its xe2x88x923 dB frequency. In examining the technology of EBS from 1948 to the present, in the literature, one can see themes and diagrams repeated but relatively little gain in performance. The fundamental physics of the vacuum triode restricts its use to below 3 GHz, with heroic exceptions to a few GHz above that. It is suitable for an arrayed amplifier structure but not higher frequencies.
Conventional TWTxe2x80x94The traveling wave tube is a mature technology with little to be added by cold cathode technology for many applications. Because cathode heater power is a few percent of overall power consumption, use of CNTs (carbon nanotubes) offers questionable advantage. Tube design is definitely simplified and the elimination of HV-isolated heater supplies is a real benefit. Considering the additional magnetics and support costs, the conventional TWT is not considered seriously here. Their size makes use in an arrayed amplifier structure difficult.
Klystronxe2x80x94The klystron is a mature and well-understood technology, composed of a series of microwave cavities. Above 20 GHz or so, fabrication of the conventional cavity arrangement becomes increasingly difficult and the device physics imposes limitations on available power. For many reasons, the conventional klystron is considered unsuitable for application as an RF array. Included in the Klystron family of devices are the Extended Interaction Klystron (EIK) and the reflex Klystron.
EIK Klystronxe2x80x94The Extended Interaction Klystron becomes more practical above 20 or 30 GHz than a typical klystron. The cavities in an EIK are multiple wavelengths in size, making them physically larger. The wavelength-related mode-enhancing structures in the cavity are still tiny but are simpler to fabricate than are mid-sized cavities. The EIK has beam and focus restrictions that dictate the use of bulky magnetics, however. For a number of reasons, the EIK is also unsuitable for an RF array.
The reflex klystron is a relatively simple and compact oscillator that uses only a single cavity and a repeller or reflector to focus the accelerated electron beam back onto itself through a small gap in the walls of the resonator. Although this concept is largely ignored in current device designs because of poor efficiency and narrow bandwidth, it may have applications in the THz range and for integrated array RF sources because of design simplicity.
Most of these examples are devices that use hot sources of electrons (thermionic cathodes). Some examples of cold cathode microwave devices exist.
The klystrode is a single stage klystron device with beam bunching accomplished by modulating the electron beam intensity from a cold cathode source (D. R. Whaley, B. Gannon, C. R. Smith, and C. A. Spindt, xe2x80x9cApplication of Field Emitter Array to Microwave Power Amplifiersxe2x80x9d, Extended Abstracts of the Int. Vacuum Electronics Conf. 2000, May 2-4, 2000, Monterey, Calif., USA, p. 9.1).
RF sources and amplifiers in the 10 GHz range currently exist. Efficiencies of 30%-50% are not unreasonable for power generation of 5-20 Watts. These devices are vacuum tube devices as solid-state technology has yet to compete in performance in this range. There are some cases where arrays of sources are required. For example, phase array sources build up a node of high intensity that can be steered by changing the phase of coherency between each of the sources. The more sources there are in the array, the sharper the node can be. Arrayed sources allow higher power operation with the ability to have large bandwidth and a high degree of redundancy. In other words, if one device in the array fails, the other sources may still operate. This is not true with a single, high power source. They also allow rapid beam steering and tracking not obtainable with mechanical changing of the direction an antenna.
The cost of array devices is presently very high. This is due to the fact that each source in the array is hand made with resulting low yields. This problem is amplified by the fact that often it is not enough that the devices operate, they must operate within tightly matched performance levels.
Efficiency and lifetime are also major concerns, especially for space-based operation due to the limited power available and understandably limited access. Cold cathode sources have been proposed for some time as a means of improving efficiency by eliminating the heater for the hot cathode source. Cold cathodes have also demonstrated high emission current densities that rival some of the best hot cathode sources. Researchers have documented emission current densities from microtip cathodes on the order of 1-15 amps/cm2 (1.2 amps/cm2 at 70V; 12 amps/cm2 at 250V). Despite these results, few examples exist of the marriage between microtip cathodes and microwave devices. The best examples of RF devices and cold cathodes are RandD prototype devices. No RF product exists that includes a microtip cathode. The issues impeding implementation are life of the cathode and its susceptibility to arc damage. Microtip cathodes generally do not die gracefully, but suddenly and unexpectedly. As a result, a large degree of skepticism has developed in the microwave community towards cold cathode technology. In both cases sited earlier, significant efforts were made in the electron gun and microwave device design and fabrication to protect the microtip source from ion feedback and arc damage, widely accepted methods of microtip failure. It is true also that hot cathode sources have limited lifetimes as well, but they are much more graceful and predictable, allowing system designers and operators the opportunity to incorporate redundancy and service schedules. Even if maintenance is not possible such as in a satellite, scheduled failure is much more acceptable than unpredictable failure.
The effort to incorporate microtip cathodes into microwave devices may be a bit misplaced. For many microwave devices, the efficiency of the cathode contributes little to the efficiency of the device. The exception is for small, low power devices and especially for device arrays.
There exists a need to make a phase array microwave device in which the array elements are fully integrated into a single device.